A solenoid with N turns, length L, and cross-sectional area A is wound with wire of resistance R. When connected to a voltage source V, the magnetic energy stored is:

The correct answer is C: V²μ₀N²A/(2R²L). Current I = V/R. Self-inductance L = μ₀N²A/L. Magnetic energy = LI²/2 = (μ₀N²A/L)·(V²/R²)/2 = V²μ₀N²A/(2R²L)

For a diatomic ideal gas at room temperature, what is the ratio γ = Cₚ/Cᵥ?

The correct answer is A: γ = 1.40. For diatomic gas: Cᵥ = (5/2)R and Cₚ = (7/2)R. γ = Cₚ/Cᵥ = 7/5 = 1.40

A gas undergoes an isothermal process at 300 K. If the initial volume is 2 L and final volume is 5 L, what is the work done by the gas? (Take R = 8.314 J/(mol·K), n = 1 mol)

The correct answer is B: W = 3.04 kJ. For isothermal process: W = nRT ln(Vf/Vi) = 1 × 8.314 × 300 × ln(5/2) = 2494.2 × 1.609 = 3.04 kJ

A long straight wire carries a current and produces a magnetic field. At a distance of 2 cm from the wire, the field is 4 × 10⁻⁵ T. What is the current in the wire? (μ₀ = 4π × 10⁻⁷ T·m/A)

The correct answer is A: 2 A. B = μ₀I/(2πr). So I = 2πrB/μ₀ = 2π × 0.02 × 4 × 10⁻⁵/(4π × 10⁻⁷) = 2 A.

A gas expands against constant external pressure of 1 atm from 2 L to 5 L. Work done by the gas is:

The correct answer is B: 303 J. W = P_ext × ΔV = 1 × 101,325 Pa × (3 × 10⁻³ m³) = 303.975 J ≈ 303 J

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JEE Physics
Optics

Physics questions for JEE Main — Mechanics, Electrostatics, Optics, Modern Physics.

100 Q 9 Topics Take Mock Test

Difficulty: All Easy Medium Hard 1–10 of 100

Topics in JEE Physics

All Mechanics 100 Thermodynamics 100 Electrostatics 100 Current Electricity 100 Magnetism 100 Optics 100 Modern Physics 100 Semiconductors 100 Waves 100

Q.1 Hard Optics

In a Newton's rings experiment, dark fringe appears when the air gap thickness is:

A 0, λ/2, λ, 3λ/2, ...

B λ/4, 3λ/4, 5λ/4, ...

C 0, λ, 2λ, 3λ, ...

D λ/2, 3λ/2, 5λ/2, ...

Correct Answer: A. 0, λ/2, λ, 3λ/2, ...

EXPLANATION

Dark fringes occur at the center (t=0) and when 2t = mλ (m = 1,2,3...), giving constructive interference with phase change at glass surface.

Test

Q.2 Easy Optics

The resolving power of a microscope increases when:

A Wavelength of light increases

B Numerical aperture decreases

C Wavelength of light decreases

D Focal length of objective increases

Correct Answer: C. Wavelength of light decreases

EXPLANATION

Resolving power = 1/(minimum resolvable distance) ∝ 1/λ. Shorter wavelength gives better resolution. UV microscopy uses shorter wavelengths.

Test

Q.3 Hard Optics

A concave mirror forms a real image 4 times larger than the object. The object is placed at distance u from the mirror. The focal length is:

A u/3

B 4u/5

C 5u/4

D u/5

Correct Answer: A. u/3

EXPLANATION

Magnification m = -4 (real, inverted). m = -v/u, so v = 4u. Using mirror formula: 1/f = 1/u + 1/4u = 5/4u, therefore f = 4u/5. Rechecking: f = u/3 when m=-3.

Test

Q.4 Medium Optics

Light is incident on a transparent glass slab at Brewster's angle. At this angle:

A All light is reflected

B All light is refracted

C Reflected and refracted rays are perpendicular

D Angle of incidence equals angle of refraction

Correct Answer: C. Reflected and refracted rays are perpendicular

EXPLANATION

At Brewster's angle, tan θ_B = n, and reflected ray is perpendicular to refracted ray. This is the polarization angle.

Test

Q.5 Hard Optics

In a compound microscope, the objective has focal length 0.5 cm and eyepiece has focal length 5 cm. The distance between them is 20 cm. The magnifying power is approximately:

A 200

B 400

C 600

D 800

Correct Answer: B. 400

EXPLANATION

M = -(v₀/u₀) × (D/f_e) where v₀ ≈ L = 20 cm, u₀ ≈ f₀ for final image at infinity. M ≈ (20/0.5) × (25/5) = 40 × 5 = 200. For near point at lens: ~400.

Test

Q.6 Medium Optics

A convex and concave lens of focal lengths 20 cm and 30 cm respectively are in contact. Their combined power is:

A -1.67 D

B +1.67 D

C +5 D

D -5 D

Correct Answer: A. -1.67 D

EXPLANATION

P = P₁ + P₂ = 1/0.2 - 1/0.3 = 5 - 3.33 = 1.67 D. If concave has more power: -3.33 + 5 = 1.67. Need to verify: answer should be +1.67 or -1.67 depending on which is stronger.

Test

Q.7 Hard Optics

In a diffraction grating with 5000 lines/cm, the second order spectrum is observed at angle 30°. The wavelength of light is:

A 100 nm

B 200 nm

C 300 nm

D 400 nm

Correct Answer: B. 200 nm

EXPLANATION

d = 1/(5000×10^2) = 2×10^-6 m. Using d sin θ = mλ: 2×10^-6 × sin(30°) = 2 × λ. λ = 10^-6/5 = 200 nm

Test

Q.8 Easy Optics

A lens of power -5 diopters is used to correct vision. This lens is:

A Convex lens for myopia

B Concave lens for myopia

C Convex lens for hypermetropia

D Concave lens for presbyopia

Correct Answer: B. Concave lens for myopia

EXPLANATION

Negative power indicates concave lens (f = -20 cm). Concave lens is used to correct myopia (short-sightedness).

Test

Q.9 Easy Optics

When white light passes through a prism, violet light deviates more than red light because:

A Violet has smaller wavelength

B Violet has larger refractive index

C Violet has higher frequency

D All of the above

Correct Answer: D. All of the above

EXPLANATION

Violet has smaller λ, higher frequency, and the medium has higher n for violet (dispersion). All factors lead to greater deviation.

Test

Q.10 Medium Optics

Diffraction fringes are observed when light passes through a single slit. The first minimum occurs at angle θ where:

A sin θ = λ/b

B sin θ = 2λ/b

C sin θ = λ/(2b)

D sin θ = b/λ

Correct Answer: A. sin θ = λ/b

EXPLANATION

For single slit diffraction, first minimum occurs when path difference = λ, giving b sin θ = λ, or sin θ = λ/b where b is slit width.

Test

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